Metazoan mitochondrial DNA (mtDNA) is a closed circular molecule of approximately 16 kbp. This small genome normally encodes for 37 genes. Although mt-gene arrangements are usually conserved, genomic rearrangements occasionally occur to produce variation in the gene orders. Because of the inourmous numbers of possible gene arrangments produced by the 37 genes, convergent evolution of the same gene order must be rare, suggesting that mt-gene arrangments would be good molecular marker for phylogenetic reconstruction. I determined mt-genome structure for various metazoans, mainly mollscs, and examined phylogenetic utility of gene order data.My study focuses on gastropod which exhibit high level of variability in the mt-genome structure. I determined complete mt-genome for a prosobranch Nordotis gigantea, a opisthobranch Pupa strigosa, two heterostrophans Cingulina cingulata and Valvata hokkaidoensis and partial sequence of many pulmonate, opisthobanch, and caenogastropodan taxa. The present study demonstrated extensive genomic rearrangments to be occurred within gastropods. Based on cladistic analysis of gene order data using the polyplacophora Katharina tunicata and the arthropoda Drosophila melanogaster as outgroups, I could reconstruct gastropodan phylogeny. The proposed phylogentic tree was consistent with recent anatocimal data, indicating that mt-gene order data is useful for phylogenetic reconstruction. I also found several changes in the gene order to be occurred within a land snail superfamily Helicoidea. Althgough gene order data has been regarded as molecular marker for distantly related taxa, present data shows that such kind of data is also useful for phylogenetica analysis of much closer taxa.